A progressive multifocal lens is a lens consisting of two portions having different refractive powers from each other, and another portion formed between the two portions and having refractive powers which is progressively changed. The progressive multifocal lens has no boundary, exhibits excellent appearance and enables fields of vision having different refractive powers to be obtained from one lens. Therefore, the progressive multifocal lenses have been used widely for lenses for eyeglasses for correcting presbyopia. FIG. 1 shows the general structure of a progressive multifocal lens which has been used widely as a lens for eyeglasses. FIG. 1 is a perspective view of an outer surface (the surface) 2 of a lens 1 for eyeglasses which projects outwards to form convexity. The lens 1 has an inner surface facing a wearer's eye and having a substantially constant curvature, and the outer surface 2 having different curvatures so as to form two portions having different refractive powers from each other and an intermediate portion connecting the foregoing two portions to each other.
When a portion extending along main meridian curve M located substantially in the central portion of the surface 2 of the lens is examined, the main meridian curve M1 above distance design reference point A has a substantially constant radius of curvature R1. Main meridian curve M2 below near design reference point B located below the distance design reference point A has a substantially constant radius of curvature R2 which is smaller than the radius of curvature of the main meridian curve M1 located above the main meridian curve M2. Main meridian curve M3 extending from the distance design reference point A to the near design reference point B has a radius of curvature which is gradually changed from R1 to R2. A portion above the distance design reference point A is a distance portion 5 having a relatively small refractive power. On the other hand, a portion below the near design reference point B is a near portion 6 which is a portion having relatively large refractive power. Assuming that the refractive power; index of the lens 1 is n, the base curve of the surface 2 of the lens is expressed by ((n-1)/R1). A major portion of the curved surfaces forming the distance portion 5 is defined by the foregoing curve. The foregoing base curve is hereinafter expressed with the refractive power (diopter (D) units) of the curved surface defined with the base curve.
A portion from the distance portion 5 to the near portion 6 is an intermediate portion 7. In this portion, the refractive power is progressively changed. The change in the plane refractive power taking place from the distance design reference point A to the near design reference point B is called addition power. Also the addition power is hereinafter indicated in diopter (D) units.
A major portion of the progressive multifocal lenses is designed such that the curvature of the main meridian curve M is the same as the curvature in a direction perpendicular to the main meridian curve M of the surface 2. That is, the portion along the main meridian curve M is formed into a substantially spherical shape having the surface, the astigmatism (the surface astigmatism) of which is zero. However, the other portions, forming one smooth surface by continuously connecting the distance portion 5 and the near portion 6 having different refractive powers to each other, are formed into aspheric surfaces. Thus, astigmatism is generated. In general, an aspheric surface is easily formed as the distance from the main meridian curve M is lengthened, thus causing the surface astigmatism to be enlarged in the periphery of the lens. Then, the surface astigmatism is obtained from the difference in the curvature at various points of the surface 2 of the lens, the obtained surface astigmatism being expressed as values converted into diopter units. A usual wearer of eyeglasses is able to obtain a clear sight if the astigmatism is 1.0 diopter or less, preferably 0.5 diopter or less, in such a manner that the wearer does not suffer from lack of focus. Therefore, the portion in which the astigmatism is included in the foregoing value range is called a "clear sight portion". The wider the foregoing clear sight portion is, the clearer images can be.
The progressive multifocal lens has astigmatism because of the structure thereof. Therefore, it is an important consideration in designing the lens to determine the permissible positions and degrees of astigmatism produced by the lens. If one point is observed closely (where static sight is required) for example reading a book, it is preferable that the overall astigmatism is small to obtain a clear image. Moreover, it is preferable that a wide clear sight be realized in each of the distance portion, near portion and the intermediate-sight portion. If an object in one's field of vision is moved relatively (where dynamic sight is required) to the field of vision, for example, where a wearer of eyeglasses follows a moving object or the wearer moves his/her neck, it is preferred that the astigmatism produced in the periphery of the lens, of course, be small. Moreover, it is preferable that change in the astigmatism is moderate and the astigmatism is routinely changed in the direction in which the user's eye moves. That is, when positions having the same astigmatism are connected by a curve, the smoother the astigmatism curves are, movement of the image can be prevented and clearer sight can be obtained.
It is very difficult to obtain a progressive multifocal lens exhibiting both excellent static sight and an excellent dynamic sight. That is, to obtain excellent static sight, a wide clear field of vision is required, thus causing the astigmatism to be enlarged in the intermediate portion and the periphery of the lens. As a result, smooth astigmatism curves cannot be obtained. Thus, excellent dynamic sight cannot be easily obtained. If the astigmatism is dispersed to obtain an excellent dynamic sight, clear field of vision is reduced. Thus, the static sight deteriorates. In order to obtain clear sight with the progressive multifocal lens of the foregoing type, a plurality of different design concepts have been disclosed. For example, a progressive multifocal lens has been disclosed in Japanese Patent Laid-Open No. 57-94714 in which the clear field of vision in the near portion is reduced because dynamic sight is not necessarily required when, for example, reading a book; and the clear field of vision in the distance portion, in which dynamic sight is required, is enlarged. In Japanese Patent Laid-Open No. 2-248920, a progressive multifocal lens is disclosed in which the clear field of vision in the near portion is enlarged and the clear field of vision in the distance portion is reduced.
In Japanese Patent Laid-Open No. 59-48732, a structure has been disclosed in which the boundary lines of the intermediate portion perpendicular to the main meridian curve are moved vertically to prevent change in the astigmatism in the side portions of the intermediate portion. In Japanese Patent Laid-Open No. 60-61719, a structure has been disclosed in which the progressive zone is considerably lengthened to prevent change in the refractive power so as to improve the intermediate field of vision and distance field of vision.
In accordance with the various design concepts, the distribution of the refractive power over the surface of the lens and the curvatures corresponding to the refractive powers are determined. Then, a curved-surface equation is obtained which is capable of smoothly connecting the foregoing portions to determine the surface shape of the progressive multifocal lens. A polynomial of degree "n" is usually employed as the curved-surface equation. For example, employment of a polynomial of ten to twenty-four degrees has been disclosed in Japanese Patent Laid-Open No. 61-252526. To express a complicated plane of the progressive multifocal lens, a curved-surface equation ten degrees or higher is required. However, even if the foregoing curved-surface equation of such higher degrees is employed, the surface of the progressive multifocal lens cannot satisfactorily be represented. In a case where smooth astigmatism curves are obtained for example, it is preferable that the surface is represented by a curved-surface equation without higher degree terms, if possible. However, if higher degree terms are not introduced, a surface of a lens in which the refractive power is distributed in accordance with the foregoing design concept cannot easily be obtained. If the addition power is large, a curved-surface equation using higher degree terms is required. If the surface of the lens is expressed by a higher degree curved-surface equation, the astigmatism is increased in the periphery of the lens. Since smooth astigmatism curves cannot be obtained in this case, movement of the image is intensified. Therefore, a curved-surface equation of intermediate degree is inevitably employed. Thus, the intention of the various design concepts cannot satisfactorily be realized. In particular, a curved-surface equation for forming the surface of a lens having satisfactory performance cannot easily be obtained for a progressive multifocal lens having a large addition power, which easily produces the astigmatism. Therefore, an appropriate curved-surface equation cannot easily be obtained if a considerably long period of time is taken.
After the higher degree curved-surface equation has been obtained, the performance of the progressive multifocal lens having the surface shape expressed by the obtained curved-surface equation is confirmed. At this time, correction of a portion of the refractive power distribution is sometimes required to improve the performance of the lens if the obtained performance is unsatisfactory. However, the obtained curved-surface equation is an equation for representing the overall surface of the lens. Therefore, partial change of the refractive power distribution does not result in a partial change of the curved-surface equation but results in change in the overall curved-surface equation. Therefore, a special curved-surface equation must be obtained. Accordingly, more time is required to again obtain a curved-surface equation. The surface shape of the lens obtainable by correcting a portion of the same is innovative, thus resulting in a necessity in which confirmation of the performance and the like of the lens is repeated. Therefore, an appropriate surface shape of the lens cannot easily be obtained and a long period of time is required. Since the curved-surface equation is determined by the foregoing method, an optimum curved-surface equation cannot always be obtained and the astigmatism cannot easily be optimized.
Accordingly, an object of the present invention is to provide a progressive multifocal lens capable of setting an optimum quantity of astigmatism in accordance with the foregoing various design concepts and a manufacturing method therefor. Moreover, an object of the present invention is to provide a progressive multifocal lens capable of easily correcting a portion of the surface shape of the lens and obtaining an optimum surface shape of the lens in a short period of time and a manufacturing method therefor. Another object of the present invention is to provide a progressive multifocal lens exhibiting a large clear field of vision and smooth astigmatism curves and enabling clear static sight and dynamic sight simultaneously and a manufacturing method therefor.